Mitochondrial genome sequencing of the monogenean Heterobothrium okamotoi isolated from the tiger puffer Takifugu rubripes in North China.

In this study, monogenean Heterobothium okamotoi was isolated and identified from the gill of diseased Tiger puffer (T. rubripes) at an industrial farm in Liaoning, North China (121.3459 E, 38.9861 N). With the completion of H. okamotoi mitochondrial genome sequencing, the full-length mitochondrial genome of H. okamotoi was assembled and analyzed. All results indicate that the complete mitochondrial genome of H. okamotoi was 14,643 bp. There were 2 rRNAs, 20 tRNAs, and 12 protein-coding genes (PCGs) all located at the heavy (H) strand. Besides, the phylogenetic tree of 19 monogeneans was constructed. The results showed that H. okamotoi and Pseudochauhanea macrorchis were clustered in a clade. To sum up, our research results would further provide essential data for systematics and evolution study of H. okamotoi.

Tiger puffer (Takifugu rubripes) belongs to the family Tetraodontidae, genus Takifugu. Owing to the fast growth rate and high economic value, T. rubripes has already become an important cultured marine fish in China (Gao et al. 2011; Jia et al. 2018). In recent years, with the fast development of tiger puffer factory farming, high-density breeding and low-quality water environment resulted in the frequent occurrence of various diseases, such as bacteria, viral, and parasitic infection (Ishimatsu et al. 2007; Mohi et al. 2010; Li et al. 2018, 2019), to cause severe economic damage and food safety risk. Among these pathogens, the monogenean Heterobothium okamotoi was one of the notorious parasites, caused acute death in T. rubripes (Ogawa and Inouye 1997a; Ogawa 2016). Some studies reported the ecology, taxonomy, pathology and, immunology, etc. of this parasitic pathogen (Ogawa 1997; Ogawa and Inouye 1997a, 1997b; Wang et al. 1997; Igarashi et al. 2017). However, no report has been made on its complete mitochondrial genome.

In the current study, H. okamotoi was isolated and identified from the gill of diseased Tiger puffer (T. rubripes) at an industrial farm in Liaoning, North China (121.3459 E, 38.9861 N). Meanwhile, the mitochondrial DNA and specimen of H. okamotoi (Number: TRHK01) were preserved and stored in the Dalian Key Laboratory of Marine Animal Disease Control and Prevention, Dalian Ocean University. With the completion of H. okamotoi mitochondrial genome sequencing by SC Gene Company (Guangzhou, China) via Illumina MiSeq Next-generation sequencing technique, the full-length mitochondrial genome of H. okamotoi was assembled and submitted to the GenBank database (No. MK948930). All analytical results indicate that the complete mitochondrial genome of H. okamotoi was 14,643 bp. There were 2 rRNAs (16S rRNA and 12S rRNA), 20 tRNAs (tRNA-Leu, tRNA-Tyr, tRNA-Ser, tRNA-Arg, tRNA-Glu, tRNA-Gln, tRNA-Phe, tRNA-Val, tRNA-Ala, tRNA-Asp, tRNA-Asn, tRNA-Pro, tRNA-Ile, tRNA-Trp, tRNA-Thr, tRNA-Lys, tRNA-Gly, tRNA-Met, tRNA-His and tRNA-Cys), and 12 protein-coding genes (PCGs) all located at the heavy (H) strand. Among these PCGs, 10 genes (nd6, nd5, cytb, nd4l, nd4, atp6, nd1, cox1, cox2, and cox3) were with start codon ATG, the rest PCGs nd2 and nd3 respectively used the start codon GTG and GTA. Besides, 7 PCGs (nd6, nd5, nd4l, nd4, nd1, cox2, and cox3) used the stop codon TAA, and the remaining five genes (cytb, atp6, nd2, nd3, and cox1) were with the stop codon TAG.

Based on sequences of 12 PCGs (atp6, cox1, cox2, cox3, cytb, nd1, nd2, nd3, nd4l, nd4, nd5, and nd6), the phylogenetic tree of 19 monogeneans was constructed by maximum likelihood method. The results showed that H. okamotoi and Pseudochauhanea macrorchis were clustered in a clade (Figure 1). Moreover, H. okamotoi, P. macrorchis, Polylabris halichoeres, and Microcotyle sebastis grouped, and all just belonged to Polyopisthocotyle (Figure 1).

Figure 1.

Phylogeny of Heterobothium okamotoi. Phylogenetic tree based on nucleotide sequences of PCGs located in the mitogenome. The number of the branches denoted BI posterior probabilities.